Crosmando: When you compare that list to the number of games on Steam...
Grargar: It's still not very rare, though.
And it's not even common on GOG. Only 4 games were removed this year, one of them also removed on Steam (Scratches) and it's a possibility that the Descent games might be removed as well (or at least change the publisher's name).
Well, I suppose there would be no reason to panic yet. The changing of ownership of rights is something that happens very often; however, it doesn't mean the new owners won't continue their contract to make those titles available through GOG unless they really felt they had a reason to not do so. (Maybe they want to re-negotiate, who knows.)
However, given some of the replies in this thread, it doesn't seem like the demand for the game is that high that it gives them leverage in negotiations. I wouldn't be surprised to see it come back for a New Year's sale at 75% off just to move more of them! :)